Reverse Polish notation executor

rpn.php

<?php
function rpn($opn, $operators='+-/*') {
    $stack = []; $operands = [];
    foreach (explode(' ', trim($opn)) as $member) {
        if ($member === '') continue;
        if (strpos($operators, $member) !== false) {
            if (count($operands) < 2) {
                array_unshift($operands, array_pop($stack));
            }
            list($first, $second) = $operands;
            $result = @eval("return $first $member $second;");
            if ($result === false) {
                throw new RuntimeException;
            }
            $operands = [$result];
        } else {
            if (! is_numeric($member)) {
                throw new UnexpectedValueException;
            }
            $operands[] = $member;
            If (count($operands) > 2) {
                $stack[] = array_shift($operands);
            }
        }
    }
    if (count($operands) != 1 || count($stack) != 0) {
        throw new UnderflowException;
    }
    return $operands[0];
}

RpnTest.php

<?php

require('rpn.php');

class RpnTest extends PHPUnit_Framework_TestCase {

    /**
     * @dataProvider provider_rpn
     */
    public function test_rpn($rpn, $expected, $operators='+-/*') {
        $actual = rpn($rpn, $operators);
        $this->assertEquals($expected, $actual, "$rpn didn't match expected: $expected");
    }

    public static function provider_rpn() {
        return [
            ['1', 1],
            ['2 1 <<', 4, '+-/*>><<'],
            ['1 2 +', 3],
            ['  3  4  + ', 7],
            ['5 0 +', 5],
            ['5 8 3 + *', 55],
            ['3 4 2 * 1 5 - 2 * / +', 2],
        ];
    }

    /**
     * @dataProvider provider_rpn_throws_exception_on_error
     */
    public function test_rpn_throws_exception_on_error($rpn, $expected_exception, $operators='+-/*') {
        $this->setExpectedException($expected_exception);
        rpn($rpn, $operators);
    }

    public static function provider_rpn_throws_exception_on_error() {
        return [
            ['+', 'RuntimeException', '+'],
            ['a', 'UnexpectedValueException', ''],
            ['1 b +', 'UnexpectedValueException'],
            ['1 2 a', 'RuntimeException', 'a'],
            ['1 2 a', 'RuntimeException', 'a'],
            ['1 2', 'UnderflowException'],
            ['1 2 3 +', 'UnderflowException'],
        ];
    }
}